1. Field of the Invention
The present invention relates to a method for electrically connecting a light-transmissive circuit board such as a flexible board or a glass board with a circuit board such as a semiconductor substrate, a printed board, a glass board, a flexible board or a ceramic board.
2. Description of the Prior Art
Conventionally, the circuit boards have been mainly connected with each other by soldering. In this case, a soldered layer is formed on an electrode of at least one of the circuit boards by plating, printing or the like, and the soldered layer thus formed is heated to a temperature of about 200.degree. to 250.degree. C. and melted so that it is connected with the electrode of the other circuit board. Accordingly, the soldering method requires a parent soldering metal such as Au, Cu, Ni or the like as an electrode material. In the case in which a circuit board such as a liquid crystal display board is connected, it is hard for an ITO (indium tin oxide) film used in the liquid crystal display board to melt and directly join with a soldering metal. Therefore, it is required to separately form a parent soldering metallic layer on the ITO film. Consequently, the cost is increased. Furthermore, since solder is heated and melted to connect both electrodes with each other, a short-circuit may easily be occurred between terminals adjacent to each other if a pitch of the electrode to be connected is fine.
In recent years, there has been used a method for connecting circuit boards by means of an anisotropic conductive adhesive in order to avoid thermal damage to the circuit boards by high temperature treatment such as soldering and an increase in cost caused by using the parent soldering metal. The anisotropic conductive adhesive has conductive particles dispersed therein. If pressure is applied, the conductive particles are condensed and fixed. Consequently, the anisotropic conductive adhesive becomes conductive only in a direction of a thickness thereof, while it becomes non-conductive in other directions. An anisotropic conductive adhesive layer is formed on the surface of the electrode or terminal to be connected and then pressurized therebetween in the direction of a thickness thereof so that the electrodes or terminals are electrically connected with each other by means of the conductive particles. Furthermore, the electrodes or terminals are mechanically connected with each other by means of the adhesive.
In particular, the anisotropic conductive adhesive has often been used in connecting terminals of the liquid crystal display board in view of easiness of connection and thermal conditions.
In a method using the anisotropic conductive adhesive mentioned above, if the electrode or terminal to be connected has a pitch width of at least 150 .mu.m, it can be connected well with the electrode or terminal opposite thereto. If the electrode or terminal has the pitch width less than 150 .mu.m, it becomes conductive between the electrodes or terminals adjacent to each other because the conductive particles are dispersed in the adhesive. Consequently, the short-circuit is caused.